In recent years, with progresses of digital technologies, electronic devices such as portable information devices and information home appliances have been developed to achieve higher functionalities. With achievement of the higher functionalities of the electronic devices, non-volatile memory devices incorporated into the electronic devices have been developed to provide a larger scale, higher-dense integration and a higher speed. The uses of the non-volatile memory devices have been spreading at a higher pace.
Among them, there has been proposed a non-volatile memory device in which variable resistance elements are arranged in an array form. It is expected that the non-volatile memory device will realize a larger-scale, higher-integration and a higher-speed.
As examples of the variable resistance element, there are a variable resistance element which is writable only once and a variable resistance element which is rewritable. As the variable resistance element which is rewritable, there are two kinds of variable resistance elements. One is a variable resistance element which changes from a higher-resistance state to a lower-resistance state (hereinafter attains the lower-resistance state) and changes from the lower-resistance state to the higher-resistance state (hereinafter attains the higher-resistance state) by application of two electric pulses having the same polarity. This variable resistance element is generally called a unipolar (or mono-polar) variable resistance element. The other is a variable resistance element which attains the lower-resistance state and attains the higher-resistance state by application of two electric pulses which are different in polarity. This variable resistance element is generally called a bipolar variable resistance element.
In a memory device in which the variable resistance elements are arranged in the array form, a current steering element such as a transistor or a steering element is typically connected to each of the variable resistance elements to prevent write disturb due to a bypass current, a cross talk between adjacent memory cells, and the like. Thus, a reliable memory operation is implemented.
Generally, to enable the unipolar variable resistance element to attain the higher-resistance state and the lower-resistance state, two voltages which have the same polarity and are different in magnitude may be used. In a case where a diode is used as the current steering element, a unidirectional diode (diode having a non-linear voltage-current characteristic “ON-OFF characteristic” in a unidirectional voltage polarity, a diode which flows a current in substantially one direction) may be used. Therefore, a structure of a memory cell including a variable resistance element and a current steering element can be simplified. However, the unipolar variable resistance element has a problem that a pulse width of an electric pulse applied in resetting (attaining the higher-resistance state) is great, which makes the element operate at a slow speed.
By comparison, to enable the bipolar variable resistance element to attain the higher-resistance state and attain the lower-resistance state, electric pulses which are short in pulse width may be used, which has an advantage that the bipolar variable resistance element is operative at a higher speed as compared to the unipolar variable resistance element. However, to attain the higher-resistance state and attain the lower-resistance state, it is necessary to use two voltages which are different in polarity. In a case where a diode is used as the current steering element, there causes a need for a bidirectional diode (diode having a non-linear voltage-current characteristic “ON-OFF characteristic” in a bidirectional voltage polarity, a diode which is capable of flowing a current bidirectionally.
Patent Literature 1 discloses a cross-point memory device including a memory cell having a structure in which the bidirectional diode as the current steering element is connected in series with the variable resistance element. As the bidirectional (bipolar) diode, there are known a MIM diode (metal-insulator-metal), a MSM diode (metal-semiconductor-metal), and a varistor disclosed in Patent Literature 1.
FIG. 14 is a view showing a voltage-current characteristic of a general bidirectional diode. Hereinafter, the characteristic of the bidirectional diode and its desired performance will be described with reference to FIG. 14. The bidirectional diode such as the MIM diode, the MSM diode, the varistor, and the like exhibits a non-linear voltage-current characteristic which can be made substantially symmetric by optimizing electrode materials and a material sandwiched between electrodes. That is, it is possible to implement the bidirectional diode in which a change in a current with respect to a positive voltage applied and a change in a current with respect to a negative voltage applied are substantially symmetric with respect to an origin.
In the bidirectional diode, its electric resistance is very high in a range (range C in FIG. 14) in which the applied voltage is equal to or less than a first critical voltage (lower limit voltage in a range A of FIG. 14) and is equal to or greater than a second critical voltage (upper limit voltage in a range B of FIG. 14), and is drastically lowered when the applied voltage is greater than the first critical voltage or less than the second critical voltage. That is, such a two-terminal element has a non-linear electric resistance characteristic (steering characteristic) in which the element does not substantially flow a current when the applied voltage is equal to or greater than the second critical voltage and is equal to or less than the first critical voltage, and flows a current of a great magnitude when the applied voltage is greater than the first critical voltage or is less than the second critical voltage.
By connecting the bidirectional diode in series with the variable resistance element to configure a memory cell, it becomes possible to implement a cross-point non-volatile memory device which performs a bipolar operation and attains a higher speed.
In the variable resistance memory device, the variable resistance element is enabled to attain the higher-resistance state or the lower-resistance state by application of the electric pulse to the variable resistance element, and these resistance states are caused to correspond to data (e.g., 0 and 1), thus writing data to the variable resistance element. In this case, typically, it is necessary to flow a current with a relatively great magnitude through the variable resistance element. Hereinafter, a current required to enable the variable resistance element to attain the higher-resistance state or the lower-resistance state will be referred to as “resistance changing current.”
For example, in the memory device disclosed in Patent Literature 1, it is recited that a current with a current density which is equal to or greater than 30000 A/cm2 (about 200 μA for an electrode area of (0.8 μm×0.8 μm)) is flowed in the varistor which is the bidirectional diode when data is written to the variable resistance element.
It is required that the bidirectional diode incorporated into the variable resistance memory device be able to flow a current (allowable current, permissible current) greater than the resistance changing current. If the allowable current of the bidirectional diode is less than the resistance changing current, the resistance state of the element does not change, which causes an incorrect operation.
In a case where data is written to or read from the variable resistance element, it is required that a leak current (OFF-current) be suppressed by using the range A or B (ON-state of the bidirectional diode) in FIG. 14 for a selected memory cell and by using the range C (OFF-state of the bidirectional diode) in FIG. 14 for an unselected memory cell. If the leak current cannot be suppressed satisfactorily, it would become impossible to correctly write and read data to and from the selected memory cell.
Patent Literature 2 discloses a bidirectional Schottky diode in which a semiconductor layer comprises amorphous silicon, polycrystal silicon, InOx, ZnO, and the like, and an electrode which forms a Schottky contact with the semiconductor layer comprises precious metal and metal compound of Pt, Au, Ag, TiN, Ta, Ru, TaN, and the like, and a similar material.